In linux, I have seen this problem and it could be caused for a number of reasons:
If you install other openCL drivers (say nvidia), sometimes I needed to reinstall the AMD drivers to make them work again (could be the same problem than the next?).
If X is not configured properly to use the AMD gpu. You can configure it by running "aticonfig --initial" with root privileges.
If you are working remotely and X is not running. You can check this by using the runlevel command. If the runlevel is 3 then you can use the 'telinit 5' to start X. I recommend to check 'man runlevel' and 'man telinit' if you are not familiar with the commands (it also needs root privileges to run).
I hope this is of any help.